The present invention relates to a primer composition or, more particularly, to a primer composition suitable for use in adhesively bonding by curing a silicone rubber belonging to the class curable by heating with admixture of an organic peroxide as the curing agent on to the surfaces of various kinds of metals as well as to a method for improving adhesive bonding of a silicone rubber to a metal surface by use of the primer composition.
When a silicone rubber composition is to be firmly bonded to the surface of a metal adhesively by subjecting the uncured silicone rubber composition to thermal curing as in contact with the metal surface, it is a usual practice that the metal surface is treated with a primer in advance before being brought into contact with the uncured silicone rubber composition. Known primers used for such a purpose include alkoxysilanes having an unsaturated bond such as vinyl trimethoxysilane, vinyl tris (2-methoxyethoxy) silane, 3-methacryloxypropyl trimethoxysilane and the like, partial hydrolysis condensation products of the above named silanes or mixtures composed of the organopolysiloxane as the hydrolysis product thereof with a titanic acid ester and a metal salt of an organic fatty acid, and peroxysilanes.
The above mentioned prior art primers are not satisfactory in respect of the thermal resistance of adhesive bonding over a long period of time when, for example, the silicone rubber bonded to the metal surface is heated at a temperature of 200.degree. C. or higher prolongedly. This defect of insufficient thermal resistance is more remarkable when heating is performed in an oil bath at elevated temperatures. In particular, peroxysilanes as the primer are especially unsatisfactory in this regard since peeling off of the silicone rubber from the metal surface readily takes place when heated in a silicone fluid at 150.degree. C. or higher.